Generally, an elevator car is supported by a wire rope that is attached, at an end, to an elevator car, passed over a drive sheave, and attached, at the other end, to a counterweight. The elevator car is raised or lowered through traction developed between the wire rope and the drive sheave, which is rotated by an electric motor. Usually, the drive sheave is arranged above the elevator car and counterweight; using pulleys to direct the rope, however, the sheave may be located below the counterweight and elevator car.
Typically, the wire rope is arranged on the drive sheave in one of two basic ways. One way, generally used in low speed applications (i.e., low-rise buildings), uses a single-wrap traction arrangement by which the rope contacts about 180.degree. of a groove within the periphery of the drive sheave. To improve the traction between the rope and the groove, the groove is often shaped, e.g. with a V-cut or undercut, or provided with a plastic insert. (Such an insert is disclosed in commonly owned U.S. Pat. No. 3,279,762.) Shaped grooves and groove inserts, however, increase wear and may cause the rope to be seized within the groove. This "seizing" may cause the rope to snarl or bend excessively, and may also prevent the rope from slipping as desired when the counterweight tension is removed from the rope (i.e., when the counterweight bottoms in a shaft).
A second way of arranging the rope on the sheave, used generally in high-speed applications (i.e., high-rise buildings), uses a double-wrap traction arrangement by which the rope contacts approximately 180.degree. of each of two parallel grooves provided within the periphery of the drive sheave for increased traction. A secondary sheave under the drive sheave serves as an idler to axially displace the rope, after one wrap (180.degree.) in one groove within the drive sheave, for a second wrap (180.degree.) in the other groove in the drive sheave. (A double-wrap traction arrangement is disclosed in U.S. Pat. No. 4,030,569.) Because rope lengths used in high-rise applications are substantial, replacing the ropes is very expensive, and rope wear therefore should be minimized. The drive sheave in a double-wrap arrangement frequently utilizes U-shaped grooves and thereby avoids the excess wear and the seizing caused by plastic inserts or shaped grooves. However, the load on the drive sheave used with a double-wrap arrangement is the sum of the loads on the rope leading to the car and counterweight plus the loads leading to and from the secondary sheave. In effect, the lead is two times the sum of the loads on the rope leading to the car and counterweight. This "double loading" can lead to excessive wear on the sheave drive shaft and the bearings that support the sheave drive shaft. Moreover, the rope may wear more rapidly than in the single-wrap arrangement because it is required to bend three times (in contrast to the single-wrap arrangement wherein the rope bends one time). The rope bends when it first engages the drive sheave, straightens when it leaves the drive sheave, bends again around the idler sheave, straightens as it leaves the idler sheave and bends again as it passes about the driven sheave for the final time. The effect on the rope is roughly equivalent to continually bending a paper clip.